1. Technical Field
The present invention concerns an arrangement in connection with equipment for continuous or semi-continuous casting of metal, in particular direct mold (DC) casting of aluminium, comprising a mold with a cavity or a mold that is provided with an inlet connected to a metal reservoir and an outlet with devices for cooling metal so that an object is cast via the outlet in the form of a billet or a wire billet.
2. Description of the Related Art
Equipment of the above type is generally known and used for casting alloyed or unalloyed metal used in the further processing of the metal downstream in the production chain, for example for remelting and extrusion purposes.
A main challenge for this type of prior art casting equipment has been to achieve a segregation-free, smooth surface on the cast product. This has been particularly important for products in which the surface is not removed before further shaping. Surface segregation is assumed to be caused by two main phenomena, inverse segregation and sweating.
When the metal comes into contact with the mold, solidification begins in a thin layer. This solidification will normally take place out from the mold and in towards the center of the billet. When the metal goes from liquid to solid phase, the external volume will decrease and this must be refilled with alloyed melt from areas further in. This produces solidification that is called inverse because the segregation takes place against the solidification front. This type of segregation typically produces a thin alloyed zone under the surface of the billet that is 10-20% higher in alloy element than the normal alloy content.
The second phenomenon, sweating, occurs when the solidified shell on the outside of the billet is not in physical contact with the mold wall. Alloyed metal can then be pressed out through the solidified shell (melting up) or partially solidified shell. This solidification produces a thin, highly alloyed zone outside the original surface and a correspondingly depleted zone under the original surface.
Inverse segregation and sweating are assumed, in turn, to be affected by a number of factors such as heat transfer from billet to mold walls, the length of the contact zone between mold and billet and grain refinement and solidification morphology, etc.
Moreover, to reduce segregation, it is important, among other things, to reduce heat transfer between mold and billet, to reduce the metal level above or in the mold, to reduce fluctuations in the metal level (produces less segregation and variation in the surface topography) and to avoid periodic fluctuations in the contact area on account of varying gas pressure and volume inside hot top molds, which produce the characteristic rings seen on the surface of billets.
One method that is in daily use and can result in a billet without surface segregation is electromagnetic casting, but this method is demanding in terms of investment and control systems. With electromagnetic casting, the pressure differences over the shell are eliminated, i.e. the sweating disappears. At the same time there is no contact between metal and mold wall. Therefore, no inverse segregation zone is formed either.
Using conventional casting technology, it is possible to reduce both sweating and inverse segregation by reducing the effect of the mold's contact with the metal.
In another method for which a patent was applied for by the applicant, which is shown and described in WO 2005/000500 and in which a hot top is used with supply devices for gas and oil in the solidification area for the metal, the contact area with the mold and the heat transfer to it are reduced. Thus, a small inverse segregation zone will be obtained. In this casting method, the metal is also supplied in such a way that the metallostatic pressure is close to zero or is zero, thus eliminating sweating.